JP7110215B6 - Lighter-than-air vehicle having a shell, laminate for such shell, method of making such laminate - Google Patents

Description

The present invention relates to lighter-than-air vehicles (including vehicles such as airships, as well as balloons) having outer shells, laminates for such outer shells, and methods of making such laminates. In particular, the present invention relates to multifunctional laminates having reinforcing fiber layers and gas barrier films.

Lighter- than-air vehicles have flexible shells filled with a gas, typically He. If the vehicle does not have an internal structural framework, it is also called a blimp, and the shape of the airship (typically elongated) is maintained by internal pressure. The shell should be made of a laminate that is stable enough not to burst and acts as a gas barrier to the typically helium (He) within the shell.

There are many requirements for shell materials for high altitude, lighter than air vehicles . It must be lightweight while at the same time providing mechanical stability. It must be chemically resistant to aggressive atmospheres at high altitudes, especially atmospheric ozone. It should also be UV resistant, stable and flexible at high and low temperatures. Materials for each of these requirements are known, but their combination presents severe challenges to the development of shell materials.

Lavan et al, U.S. Pat. No. 7,354,636, assigned to Lockheed Martin Corporation, discloses a liquid crystal polymer fiber layer, such as Vectran®, a polyimide (PI) layer secured to the liquid crystal polymer fiber layer, and a polyimide (PI) layer secured to the PI layer. A laminate having a coated polyvinylidene fluoride (PVDF) layer is disclosed. These layers are fixed together with polyurethane (PU) glue. Adjacent laminates may be secured together by PVDF cover tape on the outer surface and structural tape on the inner surface. The structural tape includes a liquid crystal polymer fiber layer and a PI layer to ensure vehicle integrity. Alternative materials may include a liquid crystal polymer fiber layer and a PVDF layer disposed on both sides of the liquid crystal polymer fiber layer. It weighs about 5 ounces/square yard (170 grams/square meter). Tensile strength is on the order of 240 lbs/inch (420 N/cm).

Given that payload capacity is directly related to shell weight, it would be desirable to reduce weight while maintaining or even increasing strength.

A good weight-to-strength ratio is described in the document "Tear propagation of a High-performance Airship Envelope Material" (Maekawa and Yoshino, Journal of Aircraft Vol. 45, No. 5, Sept-Oct. 2008). The disclosed material has a weight of 157 g/m ² and a tensile strength of 997 N/cm. The laminate contains Zylon® fibers as its base fabric. Zylon® is Toyobo's trade name for a rigid rod-shaped lyotropic liquid crystal polymer. More specifically, it is a thermoset crystalline polyoxazole, poly(p-phenylene-2,6-benzobisoxazole) (also called PBO).

Zylon® (PBO) has a high specific strength compared to other commercially available high performance fibers. Zylon® yarns also have high resistance to creep elongation, making them useful for fiber reinforcement (FR) of laminate materials. However, PBO is also known to be very sensitive to photodegradation by visible light as well as UV. It has been found that the presence of moisture and oxygen accelerates photodegradation. Thus, despite the apparent advantages of high strength and low creep, this fibrous material presents other challenges when used in stratospheric airships.

Another shell material is disclosed in US Pat. No. 6,074,722 to Cuccias et al., assigned to Lockheed Martin, where the laminate consists of a fibrous layer laminated to a plastic resin material. The fibrous layer is either a woven fabric or multiple layers of uniaxial filamentary material. Laminates with woven fabrics for airship applications are also disclosed by Vogt et al., US Pat. No. 7,713,890, assigned to Milliken & Company. The integration of electronic components in the outer shell layer is disclosed in US Pat. No. 8,152,093 to Liggett et al., assigned to Lockheed Martin Corporation.

A variety of shell materials have been demonstrated in the publication “Material challenges for Lighter-Than-Air Systems in High Altitude Applications” by Zhai and Euler, American Institute of Aeronautics and Astronautics, ^AIAA 5th Aviation, Technology, Integration, and Operations Conference (ATIO ) 26-28 Sept. 2005, Arlington California). This document discusses various materials for lighter-than-air ballonet materials, particularly for gas-retaining layers as well as for load-bearing woven layers (bearing loads/stresses). These layers are bonded together by an adhesive layer. Adhesive bonding has been described with reference to polyurethanes, epoxies and acrylics. For gas-retentive layers, this document states that among various desirable properties including low gas permeability, minimal weight, good bondability, abrasion resistance and ozone resistance, low temperature flexibility is the most important parameter. It states that there is Table 5 of this document shows that ethylene vinyl alcohol copolymer (EVOH) has poor low temperature flexibility, so EVOH is typical at high altitudes. It states that the material is not suitable. For this apparent reason, this document states that polyolefins, polyurethanes, ethylene propylene diene monomer (EPDM) rubbers and silicone rubbers are the most suitable polymeric materials for the gas retaining layer.

Although the Zhai and Euler article states that EVOH is not useful for airships at high altitudes, EVOH is useful as a sandwich layer with the polyurethanes listed as potential materials in the article. Something was discovered. In this connection reference is made to the sales catalog of Eval Europe NV (a subsidiary of Kuraray Co. Ltd.), which can be found on the Internet site http://eval-americas.com/media/15453/eval Found in %20industrial%20application.pdf . This catalog describes a coextruded film structure in which EVOH resin (Eval ^TM ) is sandwiched between layers of thermoplastic polyurethane (TPU) (simply TPU/Eval ^TM /TPU) for great flexibility and excellent gas barrier properties. ), along with its suitability in cold atmospheric conditions. One proposed use for this sandwich film is as a material for stratospheric dirigibles. The catalog states that sandwiching EVOH between two layers of TPU film overcomes the disadvantage of poor low temperature flexibility of EVOH itself. However, in a lightweight outer shell material, which is essential for proper climb capability of the airship, EVOH sandwiching between TPU layers has the disadvantage of adding weight to the outer shell without optimizing the strength of the final outer shell material. .

Therefore, it seems that the optimal solution for the shell material has not yet been found. In conclusion, although there are many proposals for prior art airship hulls, there is still a strong need for improvement and optimization.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improvement over the prior art. It is another object to provide a lighter-than-air vehicle with an improved shell. Among other things, the improved laminate shell is optimized for weight versus strength as well as minimizing gas permeability. These objectives are achieved by a lighter-than-air vehicle having an outer shell comprising laminates as described in detail below.

The following abbreviations are used.
ypi(yarns/inch), 1 inch = 2.54 cm, 1 ypi = 1/(2.54) yarns/cm
tpi (torsion/inch); tpm (torsion per meter); 1 tpi=39 tpm
gsm (grams per square meter)
sqm (square meter)
UV-Vis weathering - deterioration due to exposure to UV radiation and/or visible light

The outer shell laminate comprises a laminate material as a gas barrier and load bearing structure, the laminate comprising a reinforcing fiber layer and a first ethylene vinyl alcohol (EVOH) film melt bonded to the fiber layer on one side of the fiber layer. , the EVOH is in direct contact with the reinforcing fiber layer.

The term "direct contact" means that there are no layers of other material interposed between the EVOH layer and the fiber layer. In particular, the EVOH film is not provided as part of a composite film sandwiched between two TPU layers prior to melt bonding it to the fibrous layer.

Optionally, the laminate includes a second EVOH film melt bonded to the fibrous layer on the opposite side of the fibrous layer, the EVOH of the second EVOH film also in direct contact with the reinforcing fibrous layer. In this case, the reinforcing fiber layer is sandwiched between first and second ethylene vinyl alcohol (EVOH) films that are fused on either side of the fiber layer.

EVOH has a very low helium gas permeability, which is of great utility. It is UV stable and ozone resistant. Additionally, it is heat sealable. The poor low temperature flexibility noted in the prior art was not found to be a problem in experiments when used in laminates containing EVOH as a gas barrier layer or when used alone.

In the outer shell material, a first EVOH film is melt bonded from one side onto the fibrous layer and at least partially into the interior, and optionally a second EVOH film from the opposite side of the fibrous layer onto and at least the fibrous layer. Partially melt-bonded internally. Such fusion bonding is accomplished by hot pressing the layers together. For example, temperatures in the range of 175-180°C are useful. In the laminates described in the following paragraphs, the EVOH film not only acts as an adhesive bonding the layers together, but also acts as a gas barrier. It is therefore a multifunctional layer.

Liquid crystal fibers, such as poly[p-phenylene-2,6-benzobisoxazole] (PBO), are good candidates to provide high strength and light weight for fiber reinforced (FR) layers. Such fibers are commercially available as Zylon®, as already mentioned earlier.

In some cases, it is advantageous to include at least one of twisted fibers, sized fibers, and both sized and twisted fibers to optimize strength and/or uniformity. is. Optionally, sized fibers aid in additional bonding between layers, particularly if the sizing material is compatible with the adhesive layer. An example of a sized material is certain PVA (polyvinyl acetate) fibers. Another example is the fiber marketed under the trade name Exceval. For example, sizing of fibers is accomplished by applying a sizing material as a spin finish to the fibers.

A suitable sizing agent is polyvinyl alcohol. Such agents are commercially available, for example, from the Japanese company Kuraray Co, Ltd. Good results were also obtained in experiments with hydrophobically modified polyvinyl alcohol from Kuraray under the trade name Exceval. Such polyvinyl alcohol-based sizing agents have been found to be highly elastic, chemically resistant, and comparable to EVOH. Experiments have shown that polyvinyl alcohol strengthens Zylon® yarns. It also provided good adhesion between fabric and EVOH.

For example, the first set of fibers (optionally liquid crystal fibers) of the fibrous layer are twisted while the second set of fibers (optionally liquid crystal fibers) of the fibrous layer are not twisted.

In some aspects, the two sets of fibers are arranged in different directions. For example, a first set of fibers oriented in a first direction are twisted and a second set of fibers oriented in a second direction are untwisted. For example, the first and second directions are at an angle of at least 30 degrees, such as 45 degrees, optionally perpendicular, between the directions. Good results have been achieved with twisted fibers having between 30 and 50 twists per meter.

Both balanced and unbalanced fiber layers can be useful. In some embodiments, the second set of fibers is at least twice as thick (thick) as the first set of fibers. In some embodiments, the first set of fibers has a first thread density and the second set of fibers has a second thread density that differs from the first thread density by a factor of at least two. .

For lighter-than-air vehicles where weight is important, experimental results have been obtained with fiber layers having weights between 40 and 70 g/sqm.

For example, the thickness of the EVOH layer is between 10 μm and 20 μm.

Advantageously, the laminate includes a multifunctional weatherable layer melt bonded to the first EVOH layer, the weatherable layer being on only one side of the polymer film or alternatively on both sides of the polymer film. It includes a metallized polymer film having one metal layer. If the weatherable layer has only a single metal layer, it is advantageous to orient with the metal layer toward the EVOH and melt bond onto the first EVOH film layer. In this way it is protected by a polymer, eg polyimide, oriented to the outside of the weatherable layer. Such a weather resistant layer should protect the outer shell from reactive ozone and other chemical attacks, as well as protect the reinforcing fiber layer from UV radiation. As mentioned earlier, PBO, such as Zylon®, degrades very quickly with UV light. Additionally, it acts as an additional gas barrier. A good candidate for the weatherable layer polymer is polyimide (PI). An example of an alternative material for this purpose is polyvinyl chloride (PVF). For example, the thickness of the weatherable layer is between 10 and 20 μm.

It is pointed out that outward facing polymer layers are also useful in creating strong seams between adjacent laminates.

EVOH is a good gas barrier, but hermeticity can be improved by adding a metallized gas barrier layer to the laminate on the inside of the shell opposite the weatherable layer. For this purpose, a metallized gas barrier is optionally melt-bonded to the second EVOH film. A candidate for the metallized polymer film layer is polyethylene terephthalate (PET), eg a PET layer of thickness between 4 and 8 μm.

Experiments have shown that a weight range between 90 gsm and 110 gsm can produce airtight and stable laminates, as described in more detail below. For example, the tenacity to weight ratio for the laminate was experimentally found to be about 890 kNm/kg.

In some embodiments, the fibrous layer is a woven layer having warp and weft threads. In elongated blimps, the strength required in the transverse direction of the hull is greater than in the longitudinal direction. Thus, the warp and weft optionally have filaments of different thickness and/or density. When a non-crimp fiber layer is used, it consists of multiple layers with uniaxial filaments, with different layers having different filament directions, eg, perpendicular. Also in this case, the filaments in one direction are advantageously thicker and/or denser than in the second direction to optimize strength without adding unnecessary weight.

Both balanced and unbalanced structures have been found to be useful. The choice of balanced or unbalanced structure is objective. For example, a higher strength in one direction than the other is desirable. This relates to dimensional stability of the shell as well as weight minimization. Because of the elongated shell shape, given that the forces on the fabric are different in the longitudinal direction than in the transverse direction, an unbalanced structure is typically more flexible for optimum strength/weight ratio. This is because it has great potential.

As can be seen from the above, the laminate includes multiple layers that are multi-functional depending on the combination of each layer. Its functions include protection against UV radiation, visible light, ozone, singlet oxygen and heat. The outermost layer has a low emissivity and also provides thermal management.

For ranges between the first value and the second value, the first value and the second value are optionally included.

The invention will be explained in more detail with reference to the drawings.
FIG. 1 is a principle sketch of a laminate of shell materials. FIG. 2 is another principle sketch of a stack of shell materials. Figure 3 shows fiber reinforced (FR) layers, a) non-crimp 2-ply cross-ply, b) woven; c) non-crimp 3 Indicates ply. Figure 4 Prototype P3, a) schematic design and b) photographs of outside and inside. Figure 5 Prototype P4, a) Schematic design, b) Photos of outside and inside. Figure 6 Prototype P9, a) schematic design and b) photographs of outside and inside. Figure 7 Prototype P10, a) schematic design and b) photographs of outside and inside. FIG. 8 is a strength and weight comparison of the novel laminate prototype and laminate materials developed in other studies. FIG. 9 is a comparison of the strength-to-weight ratio of the novel laminate prototype and laminate materials developed in other studies. Figure 10 is a comparison of the tensile strength of the prototype before and after thermal exposure and accelerated UV-Vis weathering. Figure 11 is the intensity loss after thermal exposure and accelerated UV-Vis weathering. FIG. 12 is a schematic diagram of a cut-slit tear test piece. Figure 13 shows the load elongation curve of prototype P4 subjected to a constant load of 1250N. FIG. 14 shows a schematic design of prototype P12. FIG. 15 shows a schematic design of prototype P13. FIG. 16 shows a schematic design of prototype P14. Figure 17a shows the DMA measurements performed on the prototype P4 EVOH film. Figure 17b shows the DMA measurements performed on the prototype P4 Mylar film. Figure 17c shows the DMA measurements performed on the prototype P4 PI film. Figure 17d shows the DMA measurements performed on the prototype P4 warp yarns.

In order to provide an outer shell material laminate that is thin and lightweight, while at the same time being hermetic, UV-resistant, thermally and chemically resistant, especially to singlet oxygen and ozone, We used the following basic scheme: A load-bearing fiber reinforced (FR) layer is sandwiched between two adhesive layers, with which the FR is attached to additional layers, such as an outer shell layer, hereinafter so-called weather resistant layer, and Bonded to the inner shell layer with potential as a gas barrier. In particular, the adhesive layer is configured as an effective gas barrier with low gas permeability. For example, these adhesive layers are the primary gas barriers and the overall gas permeability of the adhesive layers is less than the gas permeability of the remaining layers. This combination of adhesion and low gas permeability features is unprecedented. In contrast, in the prior art, there is a specific primary gas barrier that is different from the adhesive layer, the primary gas barrier having a lower gas permeability than the adhesive layer. In this context, gas permeability relates to gas inside the shell, typically helium or hydrogen gas.

As an example detailed in FIG. 1, a load-bearing fiber layer is sandwiched between two EVOH layers, which not only act as a gas barrier, but also potentially add layers, especially the outer shell layer. , the weather resistant layer referred to below, and a possible inner shell layer as an additional gas barrier. An additional function of these layers, in particular the weatherproof layer, is protection against UV radiation, visible light, ozone, singlet oxygen and heat.

Although the optimized process used two EVOH film layers, a single EVOH layer melt bonded to the fiber layer is also believed to be advantageous over prior art laminate systems.

A variation of the design concept of FIG. 1 is shown in FIG. In this case the laminate material does not have a separate inner main gas barrier layer. Alternatively, the inner surface of the laminate is metallized after the laminate is made. The use of EVOH as an adhesive material with low gas permeability and the metallization of the inner surface gives the laminate material excellent gas barrier properties. Omitting the gas barrier layer reduces the overall laminate weight, but does not affect the tensile strength of the laminate.

Materials and methods of manufacture are described below in connection with experiments conducted to optimize the outer shell laminate materials.

Yarn Selection To simultaneously optimize low weight and high strength, it is advantageous to use high strength fibers. An example of an advantageous material for the load-bearing fiber layer is crystalline PBO (crystalline polyoxazole, poly (p-phenylene-2,6-benzobisoxazole) fibers, especially those with the trademark Zylon®.These yarns also have a high resistance to creep elongation.Therefore, Zylon® yarns was selected for fiber reinforcement in the laminate materials used in the experiments.However, PBO is known to be very sensitive to photodegradation.The presence of moisture and oxygen accelerated photodegradation. A mechanism of protection should have been discovered.

Both balanced and unbalanced structures can be used in fiber reinforced layers. In some embodiments, a non-balanced structure was chosen to provide the different strengths needed in the longitudinal and hoop directions of the LTA airship.

Zylon® yarns of 99 denier, 150 denier and 250 denier yarn counts were supplied by Toyobo Co., Ltd. The yarn as supplied had zero twist (no twist) and was tested for tensile strength. The untwisted 99 denier and 250 denier yarns had average tensile strengths of 35.5 gf/denier (4.8% cv; cv=error coefficient) and 34.9 gf/denier (3.0% cv), respectively. Given the fact that adding the optimum twist (twist factor) to the yarn provides the highest tensile strength, the optimum twist factor and corresponding A series of tests were conducted to determine the ultimate tensile strength for The twist factor (TF) depends on the number of twists (twisting/inch, tpi; twisting/meter, ^tpm ; 1 tpi=39 tpm) and the yarn count. It was calculated using yarn linear density (1 denier equals 0.9 dtex)), which is a unit of yarn count.

Yarns of 99 denier (110 dtex) and 250 denier (278 dtex) had varying amounts of twist (tpi, tpm) and were tested for tensile strength. The results summarized in Table 1 show that the tensile strength of both 99 denier and 250 denier yarns at a twist factor of 10 for the 99 denier yarn, which corresponds to 7.69 tpi (303 twists/meter, tpm). ) and 4.84 tpi (190 tpm) for 250 denier yarn.

In the fiber reinforcement morphology experiments, several principles were applied for the load-bearing layer. One was a crimp-free, two-ply cross-ply fiber reinforced layer shown in Figure 3a and the other was a woven fiber reinforced layer shown in Figure 3b. The third principle was the 90/±45 degree ply shown in Figure 3c.

Film selection for laminates
EVOH (ethylene vinyl alcohol copolymer) has a very low permeability to gases, especially He, which is why it is a good candidate for the adhesive layer.

A good candidate for an outer weather resistant layer to the outside atmosphere has been found to be PI (polyimide), but other polymers such as polyvinylidene chloride (PVF) are also possible. Advantageously, the outer weatherable layer is metallized to reflect radiation and heat. To protect the metal from damage, the metal coating faces inward and is between the weather resistant layer polymer and EVOH. In this way the polymer protects the metal from the corrosive environment of the stratosphere.

Alternatively the weatherable layer is metallized on both sides. If the weatherable layer is metallized on both sides or has a metallized side exposed to the environment, it is advantageous to protect it with a corrosion resistant coating.

In some embodiments, an inner gas barrier was added on the opposite side of the multilayer to the weatherable layer, which inner gas barrier layer was a metallized polyethylene terephthalate (PET) film, such as Mylar®.

Lamination is carried out between two aluminum platens at a pressure of 285 psi = 1965 kPa and a temperature of 175-178°C for 15 minutes on a laminate with dimensions of 7.5 inch x 7.5 inch (19 cm x 19 cm) and the temperature range The best lamination results were obtained at the upper limit of . These samples were used in various types of tests described below.

However, different temperature and residence time combinations are possible. Other experiments have shown good results at higher temperatures in the range 180-200°C, lower pressures and shorter residence times. For example, a pressure of 60 psi=414 kPa and 2 seconds at a temperature of 196° C. was used.

Strength Measurements Tensile strength measurements were performed according to the Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method) ASTM D5035. Tensile strength values are reported in units of N/cm and gf/denier (mN/tex = 88.3 gf/den). The tensile strength in N/cm represents the tenacity of the sample per unit cm of width. Tensile strength in gf/denier represents the tenacity of the sample per total denier of the yarn in the load direction. Tensile strength in gf/denier was used as a normalized metric to determine how yarn strength translates to laminated/unlaminated fiber reinforcement strength.

For the non-equilibrium structure, the inferred tensile strength (calculated based on yarn strength and fiber reinforcement structural parameters) was 1033 N/cm and 516 N/cm in the warp and weft directions, respectively, as detailed below. rice field. In general, the warp direction tensile strength of all samples (except the laminated woven fiber reinforcement) was found to be close to 1000 N/cm, whereas the weft of all samples was found to be greater than 500 N/cm. was done. Corresponding parameter values ranged from 31 to 35 gf/den.

Experiments were also conducted on a balanced fabric construction based on Zylon® yarns in the fabric. Low twist (3-5 tpi) Zylon® yarns of 99 denier and density of 46-50 ypi (approximately 18-20 yarns/cm) were used for both the warp and fill of the plain weave. The Zylon® yarn was sized with polyvinyl alcohol. Tensile strength was measured in the range of 520-615 N/cm and 28-34 gf/den in the warp and weft directions with an elongation at break of 2.9-3.2%. These results are also very good for the purpose, these are the values for the fabric and not the total laminate. More specifically, the following data were measured.

In conclusion, both equilibrium and non-equilibrium structures with high strength yarns have been shown to be very useful.

Example 1 - P3
FIG. 4a shows a particular embodiment of the principle sketch of FIG. The left side of the figure shows the weight of each layer in units of g/m ² (gsm) and the right side shows the thickness (μm). A photograph of the produced laminate is shown in FIG. 4b.

The schematic design of this laminate prototype (Experiment P3) includes an unbalanced cross-ply (2-ply) no-crimp fabric as fiber reinforcement. This fabric has 250 denier Zylon® yarns in the 90 degree and 0 degree directions. The basis weight of the fabric is 48 gsm with a yarn density of 30 ypi (30 yarns/inch is about 12 yarns/cm) in the 90 degree direction and 15 ypi (about 6 yarns/cm) in the 0 degree direction, thus Gives higher strength than the 0 degree direction. The prototype design includes a 3-layer EVOH film that provides excellent adhesion and gas barrier properties. The estimated and measured weight of this laminate prototype was 111 gsm and 109 gsm respectively.

Example 2 - P4
FIG. 5a shows a particular embodiment of the principle sketch of FIG. The left side of the figure shows the weight of each layer in g/cm ² (gsm) and the right side shows the thickness. A photograph of the produced laminate is shown in FIG. 5b.

The schematic design of this laminate prototype (experiment P4) includes an unbalanced cross-ply (2-ply) no-crimp fabric as fiber reinforcement. This fabric has 250 denier PBO yarns in the longitudinal and transverse directions. The fabric has a basis weight of 48 gsm and a yarn density of 30 ypi (about 12 yarns/cm) in the 90 degree direction and 15 ypi (about 6 yarns/cm) in the 0 degree direction. To achieve a lower laminate weight, this design uses two layers of EVOH film and a bottom layer of lightweight metallized PET film (Mylar®) that acts as the primary gas barrier layer. The estimated and measured weight of the laminate prototype is 103 gsm.

Example 3 - P9
FIG. 6a shows a particular embodiment of the principle sketch of FIG. The left side of the figure shows the weight of each layer in units of g/m ² (gsm) and the right side shows the thickness. A photograph of the produced laminate is shown in Fig. 6b.

The schematic design of this laminate prototype (experiment P9) includes unbalanced woven fabric as reinforcing fibers. The fabrics each have 99 denier Zylon® yarns with 40 tpm twist in the warp direction and 250 denier Zylon® yarns with no twist in the weft direction. The fabric has a basis weight of 50 gsm and a yarn density of 40 ypi (about 16 yarns/cm) in the warp direction and 30 ypi (about 12 yarns/cm) in the weft direction. Like the prototype P4, the bottom layer is a lightweight metallized PET film (Mylar®), which acts as the main gas barrier. Successful interlayer adhesion was obtained with a lamination temperature of 175°C, but increasing the lamination temperature to 178°C improved interlayer adhesion. The estimated and measured weight of this laminate prototype was 105 gsm.

Example 4 - P10
FIG. 7a shows a particular embodiment of the principle design of FIG. The left side of the figure shows the weight of each layer in g/m ² (gsm) and the right side shows the thickness. A photograph of the produced laminate is shown in FIG. 7b.

The schematic design of this laminate prototype (experiment P10) includes an unbalanced woven fabric as fiber reinforcement. Unlike Prototype P9, Prototype P10 uses a woven fabric with twisted 40 tpm 99 denier Zylon® yarns in the warp direction and 99 denier Zylon® yarns untwisted in the weft direction, respectively. The basis weight of the fabric is 50 gsm with a yarn density of 40 ypi (about 16 yarns/cm) in the warp direction and 75 ypi (30 yarns/cm) in the weft direction. A higher ypi (yarns/inch) in the weft direction is expected to produce a more stable fabric structure with a smoother surface texture. In the first follow-up, a lamination temperature of 175°C was used, but the adhesion of the metallized PI film was poor and the lamination quality was poor. Increasing the lamination temperature to 178 C improved the lamination quality. It wasn't as good as the prototype P9, but the results were satisfactory. The estimated and measured weight of the laminate prototype was 105 gsm.

Gas barrier performance helium permeability test results (Table 2) show that both laminate prototypes P3 and P4 have helium permeability well below the target value of 132 cc/m ² .day.atm. Laminate prototype P4 has a significantly lower helium permeability compared to that of prototype P3. The lower transmission of prototype P4 is due to the presence of the metallized Mylar® layer, which also reduced the overall weight of prototype P4.

Given the same layering scheme, the helium permeability values of prototypes P9 and P10 are expected to be the same as those of prototype P4, which is preferable to P3 in terms of hermeticity.

Laminate Strength The warp and weft tensile strengths of laminate prototypes P4, P9, and P10 are shown in Table 3. The tensile strength of one specimen of prototype P3 was 1086 N/cm.

Estimated tensile strengths calculated based on yarn strength and fiber reinforcement structural parameters are 1033 N/cm and 516 N/cm in two directions. The measured tensile strength of the prototype is slightly lower than the estimated value. The lower tensile strength compared to the inferred tensile strength is attributed to the lack of perfect alignment of the yarns and non-uniformity of the yarns due to the manual experimental fabrication of laminate prototypes. These imperfections lead to uneven load sharing in the load-bearing yarns and ultimately to premature specimen failure during tensile testing. It is believed that if the laminate is manufactured in a dedicated large scale manufacturing facility, the tensile strength will be improved and similar to the theoretical value. However, it is pointed out that the experimental values differ from the theoretical values by less than 10%, which is a very successful result.

Comparison with Other Studies A comparison of strength and weight between the new laminate prototype and laminate materials developed in other studies in the literature is shown in FIG. References can be found at the end of this section in conjunction with Table 4.

A comparison of the strength-to-weight ratios of the new laminate and laminates developed in other studies is shown in FIG. The novel laminate prototype is not only significantly lighter than any other laminate of similar tensile strength, but also significantly stronger than any laminate of similar weight. The strength-to-weight ratio of the novel laminate prototype is greater than all laminates developed in other studies.

As can be seen in this comparison, the strength of laminations P3, P4, P9, and P10 is very high relative to their weight, making them suitable for use in lighter-than-air airships. However, as shown in FIG. 9, the superior tenacity-to-weight ratio is comparable to the thicker laminates, making the exemplary laminates preferable to the overall higher tenacity laminates of FIG. It is pointed out that

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10. Cao, X, and Gao, C. "Fabrication and Investigation of Envelope Materials for Stratospheric Aircraft with PBO Fabric as Load-carriers" High-tech Fiber & Application, 34(4), pp.0-5, 2009.
11. Li B, Xing L, Zhou Z, Jiang S, and Chen X., "Study on Mechanical Properties of High Performance Envelope Materials" Material Engineering, pp.1-5, 2010.

Effects of Thermal and UV-Vis Weathering Laminate prototypes P4, P9 and P10 were subjected to two different weathering conditions. In one weathering exposure, the prototypes were subjected to a thermal exposure of 80°C for 24 hours in an oven. Another weathering exposure was an accelerated exposure to the UV and visible light (UV-Vis) spectrum from about 275 to 700 nm for 170 hours (~60 days real-time exposure at 10 km altitude). The prototype was exposed in an Atlas Ci 3000+ Weather-Ometer (www.atlas-mts.com) to an irradiation level of 1.1 Watts/ ^m2 at 340nm. The temperature of the prototype was maintained at approximately 80°C during UV-Vis weathering. For UV-Vis weathering, the specimen was mounted in a metal frame and the inside of the specimen (Mylar®) was covered with two layers of black cardboard to prevent any exposure on the inside. -The specimen was mounted on round rails inside the Ometer's weathering chamber with the outside facing the UV and visible light sources.

これは、熱及び光劣化を評価するために測定基準として用いた。 The tensile strength of the prototypes was tested after each weathering exposure. Strength loss (%) is defined below.

This was used as a metric to assess thermal and photodegradation.

The average tensile strengths before and after heat exposure and UV-Vis weathering were statistically the same (statistical analysis was performed with 95% confidence t-test). It is therefore concluded that the degradation caused by thermal and UV-Vis weathering is negligible. It is important that the specimens before and after weathering exposure were taken from different replicates of the same prototype. Due to the manual manufacturing of prototypes, there is inherent variability in the copies and even between different copies of the same prototype. Some of the heat-exposed samples showed higher strength than the corresponding unexposed samples, strongly suggesting specimen variability in the samples. A comparative graph of the tensile strength of prototypes P4, P9 and P10 before and after thermal exposure and UV-Vis weathering is shown in FIG.

The strength loss (%) of prototypes P4, P9 and P10 after thermal exposure and accelerated UV-Vis weathering is shown in FIG.

The average tensile strengths before and after heat exposure and UV-Vis weathering were statistically the same (statistical analysis was performed with 95% confidence t-test). It is therefore concluded that the degradation caused by thermal and UV-Vis weathering is negligible. It is important that the specimens before and after weathering exposure were taken from different replicates of the same prototype. Due to the manual manufacturing of prototypes, there is inherent variability in the copies and even between different copies of the same prototype. Some of the heat-exposed samples showed higher strength than the corresponding unexposed samples, strongly suggesting specimen variability in the samples.

Tear Strength Measurements The tear strength of prototypes P4 and P9 was measured using cut-slit tear test method MIL-C-21189. A schematic diagram of the tear test piece is shown in FIG. A 1.25 inch cut slit was made in the center of the specimen perpendicular to the test direction. The specimen was 4 inches wide and had a test gauge length of 3 inches between grips.

The tear strength of the specimen was calculated by averaging the five highest peak loads during the tear test. Three specimens per sample were tested in the warp and weft directions. Tear strength results for prototypes P4 and P9 are shown in Table 5.

The reason for the higher tear in the warp direction of prototype P4 compared to prototype P9 is due to the difference in the denier of the warp yarns used to contrast the two constructions. Prototype P4 used 250 denier yarn, while prototype P9 used 99 denier. It is well supported from the literature that the tear load increases with an increase in the yarn breaking load.

Resistance to Creep Elongation A 1 inch wide specimen of prototype P4 was subjected to a constant load of 1250 N for 1 day on an MTS Load frame (www.mts.com) running the load frame in load control mode. The gauge length of the specimen was 3 inches (76 mm). The load elongation curve of the test is shown in FIG. After an initial elongation of 1.6%, the P4 specimen showed a very small creep elongation of 0.02%.

Other Laminates FIGS. 14 and 15 show a particular lightweight version of the principle sketch of FIG.

The schematic design of the laminate prototype (experiment P12) shown in Figure 14 uses a non-equilibrium cross-ply, no-crimp fabric or non-equilibrium woven fabric as fiber reinforcement. The 48gsm fabric is made from PBO yarn. The inner surface of the laminate (adhesive layer) is metallized after the laminate is made. The estimated basis weight of the laminate prototype is 96 gsm.

An alternative 3-ply no-crimp fabric is expected to not only further reduce the fiber reinforcement weight, but also increase the tensile strength of the laminate. An example of a 3-ply fiber reinforced laminate prototype is described below in connection with FIG. The schematic design of the laminate prototype (experiment P13) shown in Figure 15 is a 3-ply fabric with 15 ypi (about 6 yarns/cm) in the 90° direction and 11 ypi (about 4 yarns/cm) in the +/- 45° direction ( 250 denier yarn) is used. The three plies are arranged as shown in Figure 3c. The fiber reinforcement weight is equal to 41gsm. Similar to prototype P12, the inner surface of the laminate (adhesive layer) is metallized after the lamination process. The estimated weight of the laminate is expected to be 89gsm. The estimated tensile strength of laminate prototype P13 is estimated to be higher than 1000 N/cm.

The schematic design of the laminate prototype (experiment P14) shown in Figure 16 is a three-ply fabric (250 denier yarn). The fiber reinforcement weight is equal to 59gsm. The 3 plies are arranged as shown in Figure 3c. Similar to prototype P12, the inner surface of the laminate (adhesive layer) is metallized after the lamination process. The estimated weight of the laminate is expected to be 107gsm. The estimated tensile strength of laminate prototype P13 is estimated to be higher than 1550 N/cm. The strength-to-weight ratio is estimated to be close to 1400 kN.m/Kg, much higher than all other literature-developed laminate materials.

Temperature Stability Measurements To assess whether low temperatures are a problem for material flexibility, Dynamic Mechanical Analysis (DMA) was performed on a number of samples over a wide range of temperatures. The temperature range was -60°C to 100°C. In the meantime, a Q800 DMA measurement instrument, commercially available from TA Instruments, New Castle, DE19720, USA (www.TAInstruments.com) was used to measure elastic modulus loss related to inelasticity and energy dissipation, and elastic modulus loss related to elasticity. The storage modulus was measured in MPa. In addition, the ratio between these two parameters (called Tan Delta) was calculated. The measurement results are shown in Figures 17a, 17b and 17c.

The measurement results are shown below.
Figure 17a: Sample of EVOH film with dimensions of 20 x 7 x 0.0130 mm Figure 17b: Sample of Mylar film with dimensions of 22 x 7 x 0.0050 mm Figure 17c: Sample of polyimide (PI) film with dimensions of 23 x 7 x 0.0130 mm Sample Figure 17d: P4 warp elongation 3 sample with dimensions 19 x 6 x 0.1020 mm

Measurement results showed that EVOH, Mylar and PI films were stable at low temperature without exhibiting low temperature poor strength at low temperature. For EVOH films, this is surprising in view of the previously mentioned Zhai and Euler article. The Tan Delta curve suggests a phase change that does not appear to occur in low temperature films. No evidence of delamination and physical damage was observed in the prototype P4 laminate. Heating the fabric to 180°C during processing is believed to be beneficial for stability as the polymer within the fabric is crosslinked leading to the final stage.

CONCLUSION A lightweight laminate material for high altitude, lighter than air vehicle hulls has been developed and its properties improved. The new laminate prototype is not only significantly lighter than prior art laminates of similar tensile strength, but is also significantly stronger than prior art laminates of similar weight. The specific strength (strength to weight ratio) of the laminate prototype is significantly higher than the current state of the art. The laminate prototype also has excellent resistance to thermal degradation, light degradation, chemical resistance, especially single term oxygen and ozone, excellent gas barrier properties, and excellent creep elongation resistance. Additionally, the outermost film/layer also provides excellent thermal management (including low emissivity). Therefore, layered materials have a high level of versatility. This laminate design concept can be used to tailor laminate materials to lower or higher weights while largely retaining strength to weight ratio.
The following are non-limiting examples of embodiments of the present invention.
(Aspect 1)
A lighter-than-air vehicle comprising an outer shell comprising a laminate material as a gas barrier and load bearing structure, the laminate material comprising a reinforcing fiber layer and on one side of the fiber layer the fiber layer and a first ethylene vinyl alcohol (EVOH) film melt-bonded to the EVOH, the EVOH being in direct contact with the reinforcing fiber layer.
(Aspect 2)
Aspect 1. Aspect 1, wherein a second EVOH film is melt bonded to the fibrous layer on the opposite side of the fibrous layer, the EVOH of the second EVOH film directly contacting the reinforcing fibrous layer. A vehicle that is lighter than air.
(Aspect 3)
3. The lighter-than-air vehicle of aspects 1 or 2, wherein the reinforcing fiber layer comprises fibers made of liquid crystals .
(Aspect 4)
4. The lighter-than-air vehicle of embodiment 3, wherein said liquid crystal is poly[p-phenylene-2,6-benzobisoxazole] (PBO).
(Aspect 5)
5. The lighter-than-air vehicle of aspects 3 or 4, wherein at least some of the liquid crystal fibers are twisted .
(Aspect 6)
6. A lighter-than-air vehicle according to aspect 5, wherein said twisted liquid crystal fibers comprise between 30 and 50 twists per meter.
(Aspect 7)
said fiber layer comprising at least a first set of fibers and a second set of fibers, the fibers of said first set of fibers being twisted liquid crystal fibers and oriented in a first direction; 7. A lighter-than-air vehicle according to aspect 5 or 6, wherein the fibers of the second set of fibers are untwisted liquid crystal fibers and are oriented in a second direction different from the first direction.
(Aspect 8)
8. The lighter-than-air vehicle of aspect 7, wherein said first and second directions have an angle of at least 30 degrees therebetween .
(Aspect 9)
9. A lighter-than-air vehicle according to aspect 7 or 8, wherein said first and second directions are perpendicular.
(Mode 10)
10. Any of aspects 7-9, wherein the first set of fibers has a first yarn density and the second set of fibers has a second yarn density that differs from the first yarn density by a factor of at least two. or a lighter-than-air vehicle according to claim 1.
(Aspect 11)
10. A lighter-than-air vehicle according to any one of the preceding claims, wherein said fibrous layer weighs between 40g/sqm and 70g /sqm.
(Aspect 12)
11. A lighter-than-air vehicle according to any one of aspects 1-10, wherein the thickness of said first EVOH film is between 10 μm and 20 μm.
(Aspect 13)
12. A lighter-than-air vehicle according to any one of aspects 2-11 depending directly or indirectly on aspect 2, wherein said second EVOH film has a thickness of between 10 μm and 20 μm.
(Aspect 14)
13. The lighter-than-air vehicle of any one of aspects 1-12 , wherein said laminate comprises a weatherable layer melt bonded to said first EVOH film, said weatherable layer comprising a metallized polymeric film.
(Aspect 15)
15. The lighter-than-air vehicle of aspect 14, wherein the metallized polymer film comprises a metal layer, the metal layer melt bonded to the first EVOH film layer by the first EVOH film layer.
(Aspect 16)
16. A lighter-than-air vehicle according to aspect 14 or 15, wherein the thickness of said weatherable layer is between 10 and 20 [mu]m.
(Aspect 17)
16. Any one of aspects 1-15, wherein the laminate comprises a metallized gas barrier layer melt bonded to the second EVOH film on the opposite side of the laminate to the weatherable layer. lighter-than-air vehicle .
(Aspect 18)
An embodiment wherein the metallized gas barrier layer comprises a metallized polymer film melt bonded to the second EVOH film and having a metal layer on the opposite side of the metallized polymer film to the second EVOH film. 18. A lighter-than-air vehicle according to 17.
(Aspect 19)
19. A lighter-than-air vehicle according to aspect 18, wherein said metallized polymer film layer is a polyethylene terephthalate (PET) layer having a thickness of between 2 and 6 microns .
(Aspect 20)
20. A lighter-than-air vehicle according to any one of the preceding claims, wherein said laminate has a weight between 85gsm and 120gsm .
(Aspect 21)
21. A lighter-than-air vehicle according to any one of the preceding claims, wherein the laminate has a tenacity to weight ratio greater than 890 kNm/kg.
(Aspect 22)
22. A method of manufacturing a laminate material for a lighter-than-air vehicle according to any one of aspects 1-21 , comprising a reinforcing fibrous layer and a first EVOH film or said fibrous layer on one side of said fibrous layer. of a lighter than air vehicle comprising providing first and second EVOH films on either side and hot pressing together at a temperature between 175°C and 200°C to melt bond the layers. A method of manufacturing a laminate material for
(Aspect 23)
Aspect 2 and a laminate material for a lighter-than-air vehicle according to any one of aspects 3 to 21 when directly or indirectly dependent on aspect 2, said laminate material comprising 85 gsm and 120 gsm and the laminate material is made from poly[p-phenylene-2,6-benzobisoxazole] fibers and weighs between 40 gsm and 70 gsm; and First and second 10-15 μm thick EVOH films melt-bonded to the fibrous layer on either side of the fibrous layer; and 10-15 μm thick having a metallized surface melt-bonded to the first EVOH layer. Laminate materials, including polyimide films.
(Aspect 24)
24. The laminate of embodiment 23, further comprising a 4-12 μm thick metallized polymer film layer melt bonded to said second EVOH film on the opposite side of said laminate material to said weatherable layer.

Claims (21)

A lighter-than-air vehicle comprising an outer shell comprising a laminate material as a gas barrier and load bearing structure, the laminate material comprising a reinforcing fiber layer and on one side of the fiber layer the fiber layer a first ethylene vinyl alcohol (EVOH) film melt bonded to the laminate, the EVOH being in direct contact with the reinforcing fiber layer, the laminate comprising a weatherable layer melt bonded to the first EVOH film; wherein the weatherable layer comprises a metallized polymer film and the EVOH film acts as an adhesive bonding the weatherable layer to the fibrous layer and as a gas barrier. 2. The lighter-than-air vehicle of claim 1 , wherein said reinforcing fiber layer comprises fibers made of liquid crystals. 3. The lighter-than-air vehicle of claim 2 , wherein said liquid crystal is poly[p-phenylene-2,6-benzobisoxazole] (PBO). 4. A lighter-than-air vehicle according to claim 2 or 3 , wherein at least some of the liquid crystal fibers are twisted. 5. A lighter-than-air vehicle according to claim 4 , wherein said twisted liquid crystal fibers contain between 30 and 50 twists per meter. said fiber layer comprising at least a first set of fibers and a second set of fibers, the fibers of said first set of fibers being twisted liquid crystal fibers and oriented in a first direction; 6. A lighter-than-air vehicle according to claim 4 or 5 , wherein the fibers of the second set of fibers are untwisted liquid crystal fibers and are oriented in a second direction different from the first direction. 7. The lighter-than-air vehicle of claim 6 , wherein said first and second directions have an angle of at least 30 degrees therebetween. 8. A lighter-than-air vehicle according to claim 6 or 7 , wherein said first and second directions are perpendicular. 9. The method of claims 6-8 , wherein said first set of fibers has a first yarn density and said second set of fibers has a second yarn density that differs from said first yarn density by a factor of at least two. A lighter-than-air vehicle according to any one of the preceding claims. A lighter-than-air vehicle according to any preceding claim, wherein the weight of the fibrous layer is between 40 g/sqm and 70 g/sqm. A lighter-than-air vehicle according to any one of claims 1 to 10 , wherein the thickness of said first EVOH film is between 10 µm and 20 µm. Claims 1-11, wherein a second EVOH film is melt bonded to the fibrous layer on the opposite side of the fibrous layer, the EVOH of the second EVOH film directly contacting the reinforcing fibrous layer. A lighter-than-air vehicle according to any one of the preceding claims. 13. A lighter-than-air vehicle according to claim 12 , wherein the thickness of said second EVOH film is between 10 and 20 [mu]m. 14. The method of any one of claims 1-13, wherein the metallized polymer film comprises a metal layer, the metal layer being fusion bonded to the first EVOH film layer by the first EVOH film layer. A vehicle that is lighter than air. A lighter-than-air vehicle according to any preceding claim, wherein the thickness of the weather resistant layer is between 10 and 20 µm. 13. The lighter-than-air vehicle of claim 12 , wherein said laminate includes a metallized gas barrier layer melt bonded to said second EVOH film on a side of said laminate opposite said weatherable layer. 4. The metallized gas barrier layer comprises a metallized polymer film melt bonded to the second EVOH film, and having a metal layer on the opposite side of the metallized polymer film to the second EVOH film. 17. The lighter-than-air vehicle of Claim 16. 18. A lighter-than-air vehicle according to claim 17, wherein said metallized polymer film layer is a polyethylene terephthalate (PET) layer having a thickness of between 4[mu]m and 8[mu]m. A lighter-than-air vehicle according to any preceding claim, wherein the laminate has a weight between 90gsm and 110gsm. A lighter-than-air vehicle according to any preceding claim, wherein the laminate has a tenacity to weight ratio greater than 890 kNm/kg. A method of manufacturing a laminate material for a lighter-than-air vehicle according to any one of claims 1 to 20, comprising providing a reinforcing fibrous layer and a first EVOH film on one side of said fibrous layer, A method of manufacturing a laminate material for lighter than air vehicles comprising hot pressing together at a temperature between 175°C and 200°C to melt bond the layers.

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